public class Disappear {
    public static void main(String[] args) {
        int [] arr = {3,0,1};
        int count = 0;
        for (int i = 0; i < arr.length; i++) {
            count+=arr[i];
        }
        int s = (arr.length*(arr.length+1))/2;
        if(s==count){
            System.out.println(0);
        }
        else{
            System.out.println(s-count);
        }
    }
}
